The term “radiating element” designates a combination of at least one radiating earth plane, of excitation means intended to be fed with signals, and of a resonant cavity required to radiate energy representative of these signals according to a chosen wavelength λ0.
The radiating elements used in array antennas must typically exhibit at least one of the following characteristics: high surface effectiveness and/or low bulkiness and low mass and/or the capacity to be excited in a compact manner in simple or dual-polarization and/or a bandwidth compatible with the relevant application.
The characteristic of high surface effectiveness is particularly significant when using radiating elements in array antennas, because it makes it possible to optimize the gain and to reduce the levels of the sidelobes and array lobes. Now, as is explained hereinafter, this characteristic is not easily compatible with some of the other characteristics, and notably those of compactness and integration, whatever the frequency band concerned.
The term “array antenna” designates equally well either direct-radiation active array antennas or focal array antennas, the latter having one or more focusing reflector(s), with an array of elementary sources placed in the focal zone. Such an antenna geometry is commonly designated by the initials FAFR corresponding to the conventional terminology “Focal Array Fed Reflector”. Within such an antenna, each beam or “spot” is produced by the coherent grouping of the signals of a subset of the elementary sources, with amplitudes and phases suitable for obtaining the desired antenna pattern, notably the size and the direction of aim of the main radiation lobe.
In the low frequency bands, such as for example the L or S band, the radiating elements, whatever the applications for which they are destined, are intended to deputize for overly bulky horns. The most compact horns are of Potter horn type; they have a longitudinal dimension of typically greater than 3λ0, where λ0 is the wavelength in vacuo; for example, λ0 is of the order of 150 mm in the S band. These Potter horns are limited in terms of radiating aperture, and therefore in terms of gain. Moreover large dimensions require greater lengths. Consequently, Potter horns exhibit appreciable longitudinal bulkiness, as well as large mass.
Sub-arrays, for example planar in the case of space applications, are also not satisfactory, in terms of losses and compatibility with high-power operation.
A first type of planar sub-array consists of radiating elements of patch type, linked by a triplate distributor. This distributor is relatively complex and does not easily make it possible to produce a sub-array allowing dual-polarization, or indeed dual-band operation. The losses generated in this array may also be appreciable.
A second type of sub-array, notably described in the French patent application published under the reference FR2767970, consists of the combination of an exciter resonator of patch type and of parasitic patches which constitute radiating elements known by the initials ERDV, for “Elément Rayonnant à Directivité Variable” (French for “Variable Directivity Radiating Element”). This second type makes it possible to dispense with the distributor, and therefore to noticeably simplify its definition, as well as to repolarize the fields, circularly, when the patches are chamfered and the polarization is circular. But, its implementation for apertures of greater than 1.5 times the nominal operating wavelength is complex. This concept relies furthermore on a technology of microstrip type which may be incompatible with high powers.
A simplification to the sub-arrays of the second type has been proposed. It consists in replacing, on the one hand, the parasitic patches by a metallic grid producing a semi-reflecting interface facilitating the establishment of the electromagnetic field in the cavity, and on the other hand, the exciter patch by a guided exciter, so as to define a cavity of Pérot-Fabry type, as in the case of an ERDV. The radiating element is then entirely metallic, compatible with applications requiring high power, much simpler to define than a conventional ERDV element, and makes it possible to achieve larger radiating apertures than a conventional ERDV element. However, such a radiating element possesses two drawbacks: the obtaining of radiating apertures of large dimensions requires grids of high reflectivities, so that the electromagnetic field is established in the cavity of Pérot-Fabry type. The use of these high reflectivities generates significant return of the signal to the access guide, and the matching of the radiating element is very tricky and valid only over a very narrow frequency band. Moreover, when high surface effectiveness is required, it is then necessary, in order to insert the radiating element into an array antenna, to constrain the expansion of the electromagnetic field in the cavity, by way of metallic walls. The latter induce a non-uniform distribution of the field in the metallic cavity. Admittedly, the use of grids with variable spacing makes it possible to improve the distribution of the field by causing a more significant reflection in the center than at the periphery, but then the complete structure becomes very difficult to match.
A solution is proposed in the French patent application published under the reference FR2901062. One of the embodiments presented therein, described hereinafter in detail with reference to FIG. 2, comprises a stack of two air cavities of Pérot-Fabry type, allowing great compactness, while conferring high surface efficiency as well as compatibility with signals of high power. The stack of two cavities makes it possible to relax the overvoltage coefficient of the exciter cavity, and to thus reduce the returns in the access, so as to allow better matching. However such a structure is propitious to the excitation of higher modes, notably generated by the discontinuity present at the interface of the two stacked cavities. These higher modes are detrimental to the radiation pattern of the antenna. The aforementioned patent application FR2901062 proposes to alleviate this problem through the use of lateral walls for the cavities, within which appropriate reliefs are produced. The reliefs can for example be produced in the form of longitudinal corrugations. Nonetheless, such corrugations are difficult to produce, and are relatively bulky. Furthermore, it may turn out to be necessary in practice to fill these corrugations with a dielectric, thereby rendering their production more complex, and may generate problems in a space environment, or in an environment in which it is necessary to process signals of high power.
Finally, it is necessary to associate polarization devices with antenna radiating elements. For example, the radiating elements must be able to be excited in simple polarization and/or in dual-polarization and/or in circular polarization. In a typical manner, in antennas comprising radiating elements of horn type, the dimension of the polarizer is of the same order of magnitude as the dimension of the horn. Thus, the bulkiness of the antennas is greatly impacted by the addition of polarizers.